High-temperature and pressure sulfuric acid leaching corresponding to high pressure acid leaching (HPAL) using sulfuric acid is known as a hydrometallurgy method to recover a valuable metal such as nickel or cobalt from a low-grade nickel oxide ore represented by a limonite ore, for example.
According to hydrometallurgy employing high-temperature and pressure sulfuric acid leaching, slurry produced during a manufacturing process to be discharged to the outside of a system is treated in a large-scale settling pond such as a tailings dam (slag dam). In the tailings dam, a solid in the slurry settles out using gravity and is deposited on the bottom of the dam. Supernatant water in the tailings dam is discharged to a return water pond (still pond), placed at a standstill therein, and then discharged to the outside of the system.
A thickener is known as a device for treating slurry. The thickener includes a thickener body and a rake to rotate inside the thickener body. The thickener body has a cylindrical outer frame and a conical bottom recessed deeply at the center. A solid in slurry supplied to the thickener body is flocculated, precipitated, and compressed through addition of a flocculant, gravitational settling, and agitating action by the rake. Then, the solid is deposited on the bottom. The solid is extracted from the bottom and supernatant water is extracted through an overflow line. The slurry can be treated efficiently within a relatively short period of time using the thickener.
According to hydrometallurgy using a low-grade nickel oxide ore (containing nickel of about 1% by weight in terms of grade) as a raw material, most treated ore is discharged. Thus, slurry is produced in a large quantity to result in a large quantity of supernatant water being discharged. Thus, using the thickener for treating the large quantity of slurry in the aforementioned way results in an increase in facility cost.
Thus, for construction of a hydrometallurgy plant, a land having a shape like a valley and having an area substantially the same as a production area is selected at the design stage. The exit of the valley is subjected to a process for damming up to construct a tailings dam. Further, a metallurgy facility is constructed adjacent to the tailings dam. Slurry discharged from the metallurgy facility is treated in the tailings dam.
The tailings dam is to deposit a solid in slurry by means of only gravitational settling. This requires the slurry to stay in the tailings dam for a sufficient period of time. Thus, an overflow system applied, for example, in a thickener cannot be employed for discharge of supernatant water in the tailings dam. A pump is used for discharge of supernatant water from the tailings dam to the return water pond.
However, discharging supernatant water in the tailings dam using a pump causes the following problems.
A solid in slurry is deposited on the bottom of the dam gradually, which means that a deposition height increases gradually. As the sediment gets close to an inlet of the pump, the solid is sucked into the pump, causing a failure of the pump. Additionally, the water level of the tailings dam is always changed by the quantity of water discharged. If the water level drops to expose the inlet of the pump from a liquid surface, the pump is placed in an air sucking condition of sucking air into the pump which causes a failure of the pump.
To avoid these problems, laborious and time-consuming work of always monitoring the water level of the tailings dam and the deposition height in the tailings dam and changing the position of the inlet in response to changes in the water level and the deposition height must be carried out.
Additionally, because the tailings dam and the return water pond are constructed outside, they are largely affected by change in weather conditions. During torrential rain (severe rain hard to predict by a weather forecast) that occurs frequently in a rainy season in a region having a rainy season and a dry season and also occurrs mainly in summer in Japan, rainfall of hundreds of millimeters per hour continues for several hours. The occurrence of this severe rain increases the water level of the return water pond to cause overflow from the return water pond. This causes the risk of a surrounding facility being flooded.
Patent literature 1 discloses a technique of making a submersible pump move down to follow a drop in a water level by providing a floating member to the submersible pump. The submersible pump follows a water level, so that the height of the submersible pump can be adjusted accurately.
However, this technique does not take overflow from the return water pond constructed outside that is caused by severe rain into consideration.